(A) Field of the Invention
The present invention relates to a protective circuit board and the over-current protection device thereof.
(B) Description of the Related Art
Because the resistance of conductive materials having a positive temperature coefficient (PTC) characteristic is very sensitive to temperature variation, such materials can be used for current sensing devices, and have been widely applied to over-current protection devices or circuit devices. The resistance of PTC conductive material remains extremely low at normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the resistance will instantaneously increase to a high resistance state (e.g. at least above 104 ohms), which is the so-called trip. Therefore, the over-current will be eliminated so as to protect the cell or the circuit device.
U.S. Pat. No. 6,713,210 disclosed a circuit board with over-current protection function. As shown in FIG. 1, an IC device 2 is placed on a protective circuit module (PCM) 1, and a PTC device 3 is surface-mounted on the PCM 1. The PTC device 3 is a stack structure in which a PTC material layer 6 is laminated between nickel foils (or copper foils electroplated with nickel) 7 and 7′. The nickel foils 7 and 7′ serve as electrodes for the PTC material layer 6. A nickel plate 4 serving as an external electrode is secured on the upper surface of the nickel foil 7, and a copper electrode 5 is soldered to the lower surface of the nickel foil 7′ that is adjacent to the surface of the PCM 1. The nickel plate 4 and the copper plate 5 are symmetrical with reference to the PTC device 3.
In consideration of high voltage and high current in spot-welding, the PTC device 3 cannot be subjected to spot-welding directly, and needs to be first combined with a nickel plate 4 of a thickness preferably greater than 0.3 mm, so as to avoid damage to the nickel foils 7 and 7′ of the PTC device 3 while spot-welding. However, the nickel plate 4 is usually attached to the PTC device 3 manually, which is detrimental to mass production and cost reduction.